This program on the biology of RNA tumor viruses consists of studies in three principal systems: murine leukemia virus (MuLV), murine mammary tumor virus (MTV), and feline leukemia virus (FeLV). The common objective in all of these studies is to determine the nature of the host-virus interactions leading to oncogenesis and to compare roles of exogenous and endogenous viruses in this process. The aims are to investigate: (1) Phenotypes of MuLVs and MuLV-specific macromolecules of radiation leukemias, (2) the relation of these MuLVs to induction of the TL antigen in leukemias induced by radiation or chemical treatment, (3) the role of genetic recombination in altering infectivity and leukemogenicity of MuLVs, (4) the role of endogenous MuLV-specified molecules in lymphoid cell differentiation, (5) the functions of MuLV proteins in virion formation, (6) the immune responses of cats to FeLV infection and FeLV-induced leukemogenesis, and to experimental viral vaccines, (7) the biochemical nature of FeLV- and RD114-specific cell-surface antigens (e.g., FOCMA), (8) the comparative biochemistry of FeLV RNA genomes, intracellular transcripts and proviral integration sites, (9) the detailed molecular nature of the initial RNA transcripts of integrated type-C viral genomes, (10) the biochemical relationships of horizontally and vertically transmitted MTV genomes, together with linkage studies on proviral DNA, and (11) biochemical characteristics of MTV-specific proteins and cell-surface antigens (e.g., the ML antigen). It is hoped that these studies will lead to development of new models for how RNA tumor virus genes and gene products function in oncogenesis.